1. Field of the Invention
The present invention relates to a linear amplifier for amplifying a multiple-level PSK modulated signal, which is commonly used for digital communications, at low distortion and high power efficiency.
2. Description of the Prior Art
It is desirable for transmitter amplifiers for digital radio communication systems such as cellular telephones to operate at low distortion and high power efficiency. In particular, phase distortion is of great concern. It is also desirable for the low distortion and the high power efficiency to be effected regardless of temperature changes. Such traditional linear amplifiers are explained below.
In common, field-effect transistors (FET) having a high power efficiency and thus, capable of operating at a low drain voltage are used in cellular telephones. Although such FET amplifiers serves as linear amplifiers when operated as class "A" amplifiers, they have a low power efficiency when comparing DC power consumption to output power. If the FET amplifiers operate as class "AB" amplifiers in which the power efficiency becomes high, they tend to produce considerable amounts of distortion and will hardly be suited to act as the linear amplifiers. For compensation for non-linearity, linear amplification correcting circuits have been introduced (for example, as depicted in "Linear amplification with nonlinear components", IEEE Transaction on Communications, December, 1974). However, such linear amplification correcting circuits are bulky and have a high energy consumption and will hardly be applicable to cellular telephones which should consume the lowest amount of energy.
It is known that a FET amplifier ensures a high power efficiency when the gate voltage of an FET is close to a pinchoff level under bias conditions but produces distortion at the same time. Hence, the gate voltage is predetermined for ensuring both a high power efficiency and a low distortion. As the gate voltage of the FET amplifier is varied by a change in the ambient temperature, an extra circuit is needed for determining a proper rate of the gate voltage upon measuring the ambient temperature. However, this technique causes the drain current of the FET to be unstable due to e.g. unavoidable error in the temperature measurement. Accordingly, the drain current has to be set to a greater level for averting the phase distortion, thus reducing the power efficiency.